1. Field of the Invention
The present invention relates to a phase-change optical recording medium in which atomic arrangement of the recording film is changed between amorphous and crystalline states upon irradiation with a light beam.
2. Description of the Related Art
(Principle of Phase-change Optical Recording Medium)
In recent years, a rapid propagation is being achieved in the optical recording medium, particularly, in the phase-change optical recording medium in which atomic arrangement of the recording film is changed between amorphous and crystalline states upon irradiation with a light beam. In the phase-change optical recording medium, data writing, reading and erasure are carried out according to the following principle. The writing is carried out by utilizing the phenomenon that the phase-change optical recording film generally assumes amorphous atomic arrangement when a portion thereof is heated to a temperature higher than the melting point and molten and then rapidly cooled. Since intensity of reflected light from the amorphous portion of the film differs from intensity of reflected light from the crystalline portion, the intensity of the reflected light is converted into intensity of an electrical signal and, then, the electrical signal is subjected analog-to-digital conversion so as to make it possible to read out the information (reading). Further, if the phase-change optical recording film is maintained for a prescribed time or longer in a temperature range lower than the melting point and higher the crystallizing temperature, the crystalline state of the film is maintained in the case where the recording film is crystalline, but the recording film is crystallized where the recording film is amorphous. Therefore, it is possible to bring the state of the phase-change optical recording film back to the initial crystalline state (erasure).
It should noted that the writing and reading can be performed by utilizing phase-change between a metastable crystalline phase and a stable crystalline phase as in martensite or phase-change among metastable crystalline phases as well as the phase-change between the crystalline phase and the amorphous phase noted above.
(Means for Increasing Recording Density)
The amount of information that can be recorded in a single recording medium, i.e., the recording capacity, can be increased by the two methods described below.
A first method is to shorten the pitch between the recording marks in the circumferential direction of the track, i.e., to shorten the bit pitch. In this method, however, if the pitch between the recording marks is significantly reduced, a difficulty that two recording marks are included temporarily in the read beam spot may be caused. Specifically, it is possible for. Where the recording marks are sufficiently separated from each other, the read signal is highly modulated so as to make it possible to obtain a signal having high amplitude. However, where the recording marks are close to each other, the obtained signal has low amplitude, with the result that errors tend to occur in the stage of converting the obtained signal into digital data.
Another method for improving the recording density is to reduce an interval in the radial direction of the tracks, i.e., to shorten the so-called “track pitch”. This method makes it possible to increase the recording density without being affected significantly by decrease in the signal intensity that is caused by the mark pitch reduction noted above. In this method, however, a so-called cross-erase problem is generated. Specifically, if the track pitch is made substantially equal to or smaller than a light beam size, the information in a certain track deteriorates when writing or erasure is performed on the adjacent track. The cross-erase problem is generated partly because the marks on a certain track are irradiated directly with the edge of the laser beam on the adjacent track, and partly because heat generated in writing the adjacent track flows into the track in question so as to raise the temperature of the marks on the track in question and, thus, to deform the marks. In order to increase the recording density of the phase-change optical recording medium, it is necessary to overcome the problems pointed out above. Also, in order to suppress the probability of read errors for small recording marks to a low level, it is desirable that the recording marks be formed in a manner to have an even contour so as to suppress noise components as much as possible.
(Increase in Recording Capacity by using Multi-layer Disc)
Another method for increasing the recording capacity is to stack a plurality of layers for carrying information, which is formed of a multi-layered film including a recording film and referred to as an information layer hereinafter (see Japanese Patent Disclosure (Kokai) No. 2000-322770). The recording medium that is designed such that two information layers are stacked one upon the other and the information is written to and read from one side is called a single-sided, dual-layer disc or is simply called a dual-layer disc. It is possible to stack two single-sided, dual-layer discs so as to form a double-sided, quadruple-layer disc for further increasing the recording capacity. Of the stacked two information layers, the information layer positioned close to the light incident side is called L0, and the information layer positioned remote from the light incident side is called L1. In the single-sided, dual-layer disc, it is necessary that the transmittance of the L0 information layer positioned close to the light incident side is at least about 50% to prevent the light from being attenuated excessively in the information layer L0 close to the light incident side in accessing to the information layer remote from the light incident side. Such being the case, it is necessary for the phase-change optical recording film included in the L0 information layer to be very thin, i.e., about 5 to 7 nm. In the case of such a thin recording film, the retention time required for the crystallization is prolonged, with the result that the recording mark fails to be erased completely at the normal recording speed. The particular situation will be described herein later in detail. As a measure against the difficulty, it is known that it is effective to substitute Sn for a part of the GeSbTe recording film (see Proceedings of The 12th Symposium on Phase-Change Optical Information Storage. PCOS 2000, pp. 36–41). Also, it is known that it is effective to substitute Bi, In, Sn and Pb for a part of the GeSbTe recording film (see Japanese Patent Disclosure No. 2001-232941). In contrast, it is necessary that the writing and erasure must be performed in the L1 information layer by the laser light the intensity of which has been substantially halved by the L0 information layer and, thus, which requires increase in the sensitivity of the L1 information layer.
(Manufacturing Process of Single-Sided, Dual-Layer Disc)
The manufacturing process of the single-sided, dual-layer disc can be classified into two procedures given below depending on the stage at which the initialization is performed. The term “initialization” denotes the process of irradiating an amorphous recording film immediately after deposition with an initializing beam having a relatively large width, the initializing beam having a wavelength greater than that of the write beam and intensity lower than that of the write beam, so as to crystallize the recording film. In this case, the initialization is performed over the entire region of the disc while rotating the disc and moving the initializing beam in the radial direction of the disc.
(1) A method in which two polycarbonate (PC) substrates are prepared, and a multi-layered film constituting the information layer for each of these two PC substrates is deposited, followed by initializing the recording film included in each of the information layer. Then, the two PC substrates are adhered to each other. In this method, the initialization is performed in the state that the deposited multi-layered film is exposed to the outside for each of the L0 information layer and the L1 information layer.
(2) A method in which two PC substrates are prepared, and a multi-layered film constituting: the information layer for each of these two PC substrates is deposited, followed by initializing the L1 information layer alone, and then, the L0 information layer and the L1 information layer are adhered to each other, followed by initializing the L0 information layer. In this method, the L1 information layer is initialized in the state that the deposited multi-layered film is exposed to the outside, while the L0 information layer is initialized in the state that the multi-layered film is sandwiched between the two PC substrates and, thus, is not exposed to the outside.
In the high-density optical recording medium having a reduced track pitch, it is possible that the final disc characteristics may be degraded by the initializing process, making it necessary to pay careful attentions. The above problem is caused by the following reason. That is, the films and the substrate constituting the recording medium are thermally expanded by the initializing beam. However, where the track pitch is made very small and thus the number of tracks including the lands and the grooves is increased relative to the diameter of the initializing beam, it is difficult to make the crystalline state of the recording film for each track uniform while making uniform the influences given by the thermal expansion.
The reason why at least one information layer is initialized before the L0 and L1 information layers are adhered to each other as in procedure (1) or (2) given above is because discs subjected to defective initialization, if produced, are to be removed before the adhesion stage so as to improve the yield. In the single-sided, dual-layer phase-change optical recording medium, a interlayer separating film having a thickness of about 20 to 30 μm is formed between the L0 information layer positioned close to the light incident side and the L1 information layer positioned remotely from the light incident side. In general, the interlayer separating film noted above is formed by the process including the steps of spin-coating the multi-layered film included in one of the two information layers with ultraviolet (UV) curing, adhering the multi-layered film of the other information layer so as to face the above multi-layered film, followed by irradiating the adhered structure with ultraviolet light so as to cure the resin. The interlayer separating film is required to exhibit thickness uniformity that the distribution of the thickness is not larger than ±1 μm. In order to satisfy the condition noted above, it is necessary to coat a disc having a large area of, for example, 120 mm diameter with a UV-curing resin having a reasonable fluidity, followed by uniformly irradiating the resin with an ultraviolet light so as to cure instantly the resin with a high uniformity.
In the process of reading the signal from one information layer included in the single-sided, dual-layer disc, it is possible for the reflected light or the scattered light from the other information layer to be contained in the read signal, which may raise a noise level. In some cases, it is difficult to discriminate the noise by the measurement of the carrier-to-noise ratio (CNR) alone. The noise due to the above reason is found by measurement of bit error rate (bER) and the detailed study with respect to relationship between the noise components contained in the read signal and the tracking signal and the noise level.
(Means for High-Speed Recording)
High-speed recording is also required for the phase-change optical recording medium. For example, if the writing can be performed in a time shorter than the actual viewing time, it is possible to easily realize the so-called “time-shift function” that the previous images can be viewed in the copying stage of the distributed recording medium or during the real-time recording of the broadcasting images. However, one of the factors for inhibiting the high-speed recording in the phase-change optical recording medium is a problem that the information fails to be erased completely when crystallization is performed in the overwriting stage by a laser beam at a relatively low erasure level, i.e., the problem of the insufficient erasure rate. To be more specific, since a recording mark passes through the laser spot at a high speed, it is difficult to retain the recording mark for a sufficiently long time in temperature range within which the crystallization can be performed, with the result that the information fails to be erased completely. In order to ensure a satisfactory erasing operation, the uniformity of the formed recording marks and the recording film itself is rendered more important. In order to enhance the uniformity in the shape of the recording marks, the uniformity in the initialized state, i.e., in the crystalline state is required.
(Film Design of Phase-Change Optical Recording Medium)
As described previously under the item of the principle of the phase-change optical recording medium, the information is written in the phase-change optical recording medium by irradiating a desired portion of the recording film with a laser beam, followed by rapidly cooling the irradiated portion so as to form an amorphous mark, and the written information is erased by irradiating the amorphous mark with a laser beam, followed by gradually cooling the irradiated portion so as to crystallize the irradiated portion. It follows that the writing and erasure can be performed with a lower laser power when the absorbance in the recording film is high. In contrast, a higher laser power is required for the writing and erasure when the absorbance in the recording film is low.
It should be noted that the laser beam absorbance in the recording film is determined by the optical characteristics of the information film formed of a multi-layered film. What is also important is the thermal design relating to the film structure as to, for example, whether a rapid cooling structure is employed, even if the absorbance is the same. Thus, in the film design of the phase-change optical recording medium, the optical design and the thermal design are mainly taken into consideration. For the optical design, the optical characteristics of each thin film are required. Also, for the thermal design, the thermal properties including the melting point of each thin film, the latent heat of melting, and the crystallization temperature are required.
As described above, in the single-sided, dual-layer disc, it is necessary for the recording film included in the L0 information layer to be very thin, e.g., about 5 to 7 nm, and for the recording film included in the L1 information layer not to be unduly thick, e.g., about 10 nm. In the particular film construction, the damage suffered by the recording film in the initializing stage has been increased, leading is to the requirement of the film design in view of the entire process.